1. Field of the Invention
The present invention is for an apparatus and a method for anticipating disk burst failures in turbo-machinery rotors. The apparatus comprises:
at least one blade-tip sensor for sensing blade passage and making measurements of radial, tangential (circumferential) and axial blade-tip deflection;
a computer for compilation and analysis of data from the sensors; and
optional additional sensors
2. Description of the Related Art
If they are not replaced, turbo-machinery disks (spinning hubs and blades) eventually burst due to spin-induced stresses and fatigue. The failure pattern varies, depending upon the specific crack and disk in which it grows. Moreover, such deformation is not symmetric, but rather is biased due to asymmetric crack propagation in the disk.
Historic methods to predict disk failure require disassembly of the turbo-machinery for standard materials analysis, including x-ray crystallography, eddy current inspection, sonograms, and other diagnostic techniques. The predictive value relies on estimating when micro-cracks have formed and then searching for confirmation. Due to wide variations in operating conditions and in the fatigue life of turbo-machinery components, historic methods require frequent inspection intervals to maintain safe operation.
A recent experimental technique referenced in U.S. Pat. No. 6,098,022 (Sonnichsen) employed in spin pit tests relies on changes in the imbalance when a turbo-machinery disk is spun suspended on a quill shaft. The reasoning behind this technique is that crack propagation will progressively shift the imbalance in the rotating disk. This technique found application in controlled experiments in spin pits, but may be difficult to apply in engine operation, where many effects cause shifts in imbalance.
Disk deformation can be monitored at the blade tips at the perimeter of the rotor. For example, in a rotor with N blades, a blade-tip sensing system provides N measures of rotor radius for every engine revolution. The trend of such measurements over many operational cycles can reveal a local bulge or bulges indicative of impending disk burst.
Tangential displacement of rotor blades can also be monitored at the blade tips. For example, in a rotor with N equally spaced blades spinning at P revolutions per second, equally spaced blades should arrive every P/N second. Blade arrival times that vary from P/N indicate tangential displacement.
Axial displacement of rotor blades can also be monitored at the blade tips. For example, in a rotor with radius R, spinning a speed P, blade-tip speed is 2xcfx80RP. If blade-tips form a 45 degree angle with the spin axis of the engine, an axial displacement of distance h results in a delay in blade-tip time of arrival of h/(2xcfx80RP). If the rotor is distorted axially into a saddle or xe2x80x98potato chipxe2x80x99 shape, the sensor will register a varying sine wave in time-of-arrival that corresponds to the axial deflection.
The disk deforms elastically as the disk is spun to high rpm and contracts again when the disk is slowed. Such xe2x80x9celastic stretchxe2x80x9d can create a relatively large signal, roughly equal for each blade on the spinning disk. The detection system must discount this form of deformation. One way to do so is to compare disk shape during each operating cycle at the same rpm, so as to detect only variations due to causes other than elastic stretch. Another way is to normalize all rotor deformations by a predictive model of how these deformations are expected to vary with rotor speed; the simplest such prediction is that elastic deformation varies with the square of rotor speed.
An additional complication is the variation due to temperature, which can also add significant deformation. This deformation is typically axisymmetric and can be disregarded by pattern recognition techniques.
It is an object of the present application to present an apparatus and method for anticipating disk burst failures in turbo-machinery in which one or more case-mounted sensors observes radial, tangential and axial displacement of each blade tip as a function of rotational velocity and a computer to search the data for deformation trends that indicate crack development and propagation.